Ceramic honeycomb structures are used in vehicular exhaust systems to reduce pollutants. Such structures generally have a network of interconnected walls that form a matrix of elongated, gas-conducting cells which may be square, octagonal or hexagonal in shape. This matrix has been described as a “honeycomb” matrix. For example, the network of walls may be surrounded by a cylindrical outer skin that is integrally connected to the outer edges of the walls to form a cylindrical- or oval-shaped cylindrical structure having opposing inlet and outlet ends for receiving and expelling exhaust gases through the honeycomb matrix of walls.
Such ceramic honeycomb structures may be used as either particulate filters in the exhaust systems of diesel-powered automobiles or other equipment, or as chemical filters such as automotive catalytic converters. When used as particulate filters, the open ends of the cells on the inlet and outlet ends of the structure are preferably plugged in “checkerboard” fashion such that exhaust gases entering the inlet end of the structure must pass through the porous ceramic walls before they are allowed to exit the open ends of the cells at the outlet end of the structure. When used as catalytic converters, the cells remain unplugged so that the exhaust gases may flow directly through them, and the walls defining the cells are coated with a precious metal catalyst containing platinum, rhodium, or palladium, for example. The catalyst impregnated onto and into the walls promotes chemical reactions that convert CO, NOx and hydrocarbons into non-polluting compounds such as H2O, O2 and N2. A useful measurement of the effectiveness of a catalyzed substrate is the light-off temperature—the temperature of the gas stream entering the substrate at the time when the gas stream exiting the substrate has 50% lower levels of pollutants than the entering gas stream. Both applications of ceramic honeycomb structures are important in reducing pollutants that would otherwise be expelled into the environment.
Ceramic honeycomb structures are formed by extruding a wet, paste-like, ceramic precursor to cordierite, mullite, silicon carbide, or aluminum titanate through a die to simultaneously form the network of walls preferably along with the integrally-connected outer skin. The resulting extruded green body is cut, dried and moved to a kiln which converts the green ceramic body into a fired ceramic body. The fired body may then either be plugged in the aforementioned pattern to form a diesel particulate filter, or subjected to a catalyst wash coat in order to impregnate the walls of the flow-through cells with the catalyst.
The effectiveness of a ceramic structure used as a catalytic converter is dependent in part upon the amount of intimate contact achieved between the exhaust gas and the catalyst-coated cell walls. Unfortunately, the fluid mechanics between a predominantly laminar flow of exhaust gas and the cell walls of a standard honeycomb ceramic structure are not conducive to the efficient achievement of intimate contact between the catalyst and the pollutant molecules.
One way to improve such contact would be to increase the cell density, thereby increasing both the area of initial impingement between the gas or fluid and the leading edges of the honeycomb structure, as well as the total area of the cell walls defining the flow channels. However, there are drawbacks to this strategy as increased cell density also increases both the amount and hence expense of the precious metals used as the catalyst, as well as the back pressure the converter applies to the exhaust system. Another strategy would be a more efficient use of the catalyst applied over the surfaces of the honeycomb cells. To this end, ceramic honeycomb designs have been proposed that provide holes or intersecting channels between the flow-path channels to promote better mixing, or a spiral shape to the cell walls for the same purpose.
There is a need for a more efficient honeycomb-type fluid treatment device that provides a greater amount of intimate contact between a fluid, such as an automotive exhaust gas, and the cell walls of a honeycomb structure, without increasing the back pressure that the device applies to the fluid flow and without the need for additional amounts of expensive precious metal catalysts or other treatment agents present in the cell walls.